[BUG] Potential infinite recursion on `repr(treespec)` / `hash(treespec)` when the node metadata references the `treespec` itself

Required prerequisites

[X] I have read the documentation https://optree.readthedocs.io.
[X] I have searched the Issue Tracker that this hasn't already been reported. (comment there if it has.)

What version of OpTree are you using?

0.9.2

System information

Install from source
Python version: 3.11.5

Problem description

The treespec representation calls repr / hash for the node metadata for each node. When the node metadata references the treespec itself, this will cause infinite recursion of calling repr(treespec) / hash(treespec).

Note that this situation is almost impossible to happen in the real world.

Reproducible example code

The Python snippets:

import optree

class Holder:
    def __init__(self, value):
        self.value = value

    def __eq__(self, other):
        return isinstance(other, Holder) and self.value == other.value

    def __hash__(self):
        return hash(self.value)

    def __repr__(self):
        return f'Holder({self.value!r})'

key = Holder('a')

treespec = optree.tree_structure({key: 0})
print(treespec)  # -> PyTreeSpec({Holder('a'): *})

key.value = 'b'
print(treespec)  # -> PyTreeSpec({Holder('b'): *})

key.value = treespec  # self reference
print(treespec)  # -> RecursionError
print(hash(treespec))  # -> RecursionError

Traceback

Traceback (most recent call last):
  File "/Users/PanXuehai/Projects/optree/test.py", line 27, in <module>
    print(treespec)  # -> RecursionError
    ^^^^^^^^^^^^^^^
  File "/Users/PanXuehai/Projects/optree/test.py", line 15, in __repr__
    return f'Holder({self.value!r})'
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/PanXuehai/Projects/optree/test.py", line 15, in __repr__
    return f'Holder({self.value!r})'
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/PanXuehai/Projects/optree/test.py", line 15, in __repr__
    return f'Holder({self.value!r})'
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  [Previous line repeated 246 more times]
RecursionError: maximum recursion depth exceeded while getting the repr of an object

Traceback (most recent call last):
  File "/Users/PanXuehai/Projects/optree/test.py", line 27, in <module>
    print(hash(treespec))  # -> RecursionError
          ^^^^^^^^^^^^^^
  File "/Users/PanXuehai/Projects/optree/test.py", line 12, in __hash__
    return hash(self.value)
           ^^^^^^^^^^^^^^^^
  File "/Users/PanXuehai/Projects/optree/test.py", line 12, in __hash__
    return hash(self.value)
           ^^^^^^^^^^^^^^^^
  File "/Users/PanXuehai/Projects/optree/test.py", line 12, in __hash__
    return hash(self.value)
           ^^^^^^^^^^^^^^^^
  [Previous line repeated 330 more times]
RecursionError: maximum recursion depth exceeded while calling a Python object

Expected behavior

No response

Additional context